1. Field of the Invention
The present invention relates to the packaging of semiconductor devices and particularly to that of micromechanical devices.
2. Description of the Related Art
Packaging is a critical part of producing high-performance semiconductor devices, particularly micro-electrical mechanical systems (MEMS) such as micromirror devices. Environment is a critical parameter for most semiconductor chips. For example, in a micromirror device, which is a particular variety of spatial light modulator, the mirrors land on the substrate surface of the device. In order to avoid the mirrors from sticking, it is necessary that the micromirror device is lubricated and packaged in an environment with minimal amounts of moisture, adhesives, dust, and other contaminants. It is shown in U.S. Pat. No. 5,331,454, titled xe2x80x9cLow Reset Voltage Process for DMD,xe2x80x9d that lubrication of the surface of the micromirrors landing electrodes with an oriented monolayer, such as Perfluordecanoic Acid (PFDA), can decrease the Van der Waals forces and thereby reduce the tendency for the mirrors to stick to the landing electrodes. It is also know that water of the surface of the micromirror increases the tendency of the mirrors to stick.
The packaging of micromirror chips for use in projection displays and other electro-optical applications has continued to present a cost barrier that contributes to higher priced projection display products. These packages are not only expensive, but also require a low throughput process that increases the product cycle time, thereby further driving up the cost.
FIG. 1 shows a typical micromirror package. In the past, this has often been a hermetic package, although lower-cost plastic packages have been disclosed. The illustrated package is comprised of a ceramic case 10 and an optical cover glass (lid) 11. The cover glass 11 has an aperture 110 through a light shield 120 deposited on the bottom side of the cover 11. The cover 11 is attached to the package case 10 by either welding or adhesive techniques. The primary purpose of the light shield 120 is to prevent stray light from reaching the bond wires and bond pads around the peripheral of the micromirror. These regions reflect stray light reaching them and the reflected light enters the projection optics of the display system, lowering the contrast ratio of the projected image. Packages housing non-electro-optical devices use opaque top covers 11.
The environment inside these chip packages can be largely controlled by means of getters. FIG. 2 shows a micromirror package""s top cover 20 with getter material 21 attached to the bottom surface to absorb moisture inside the package. The getter strips 21 are attached to the cover 20 with an adhesive 22.
There is a need for an improved method of placing and holding a getter and other environmental control materials in a semiconductor package. The invention disclosed herein addresses these needs by moving the getter from the package cover and providing a low-cost drop-in environmental control material carrier assembly, which holds various kinds of environmental control materials material.
This invention discloses an improved semiconductor package with a drop-in environmental control material carrier assembly that provides higher-performance, lower-cost, and longer-life devices. The disclosed approach separates the getter material from the package""s top cover and positions it in the package""s cavity by means of a drop-in environmental control material carrier assembly.
The environmental control material carrier assembly can hold various types of material and is in thermal contact with the package substrate walls. For example, in the case where the chip is a micromirror device, the frame will typically hold three types of materials; (1) a desiccant for absorbing moisture, (2) a getter for absorbing adhesive outgassing constituents, and (3) a reservoir of PFDA lubricant. This provides higher environmental control material capacity which results in overall higher performance and longer life micromirror devices.
Potential advantages provided by the disclosed invention may include:
1. the package is made up of lower cost piece parts
2. offers multi-material functionality (moisture absorption, adhesive outgassing absorption, and lubricant storage) capability,
3. provides higher environmental control material capacity, and
4. provides an easier assembly process which results in lower cost devices.